The universe around us is so enormous that it is difficult even to imagine its dimensions. In a far corner of the Milky Way Galaxy, (one of the billions of galaxies comprising the universe), sits our solar system. Within this system, Earth, the planet on which we live, is one of the planets revolving around the Sun.
The physical conditions on primitive earth were not congenial for life. The earth was extremely hot - a ball of hot gases.
The whole universe formed probably 12 to 14 billion years ago as a result of a Big Bang and subsequent expansion. Our solar system came into existence 5-7 GY ago (Giga Year). In its initial stages of formation (4.5 GY), the earth was impacted by another planet that caused the spin (that gave us day and night) and tilt (that gave us seasons) of our planet and also led to the formation of moon. For nearly 700 million years (up to 3.8 GY ago), the earth experienced frequent and catastrophic bombardment by meteorites of different sizes.
Gradually earth’s crust solidified although volcanoes kept on spewing out harmful gases. These gases accumulated and combined to form methane, ammonia and hydrogen cyanide. These three gases along with minor gases like carbon dioxide and carbon monoxide, formed the atmosphere of the primitive earth. There was no oxygen then, a gas so essential for nearly all living organisms.
There is a general belief that life on earth must have originated, not before 4.0 GY and no later than 3.5 GY ago. Some fossils (remains of living beings that once existed on earth) claimed to be cyanobacteria (blue green algae) were found in Australia from rocks dated 3.5 GY. But cyanobacteria are fairly complex and advanced and therefore we may assume that life originated much earlier than 3.5 GY. So, for the present we accept that life originated nearly 3.8 GY ago.
One theory, proposed by the British biologist J. B. S. Haldane and the Russian scientist A. I. Oparin, suggested that life originated in the shallow seas where important organic compounds (such as amino acids), the building blocks of life, were present in high concentrations (forming a "primordial soup"), thus providing the necessary ingredients for emergence of life. But where did these organic molecules come from? Haldane and Oparin suggested that in the reducing atmosphere (because of the absence of oxygen) of the primitive earth they could have formed from inorganic substances which were washed down with torrential rains as earth cooled and formed a "primordial soup" in which life originated.
Later Stanley Miller and Harold Urey provided experimental support for this hypothesis. Under laboratory conditions they successfully produced amino acids by passing an electric charge (simulating lightning) through a flask containing methane, ammonia and hydrogen in solution.
On the deep sea floor of the oceans there are sites which have vents or deep cracks through which extremely hot dissolved gases and minerals keep belching out like fountains from the earth’s interior. A special group of archebacteria thrive near these vents as they are adapted to live at high temperatures exceeding 100°C (and hence their name, hyperthermophiles), and derive energy chemosynthetically from the hot gases.
Evolutionarily, these microorganisms are very ancient (~ 3.5 GY) and probably among the earliest living organisms on earth. These observations lend support to the more recent hypothesis that life evolved around such hydrothermal vents in the oceans.
Regardless of where life had begun, how life emerged is still a mystery. Even if we assemble all the organic compounds essential for life we cannot produce from them a living organism capable of growing, reproducing and, storing and passing on a hereditary map to its offspring.
How was it possible then that life suddenly emerged in a certain "primordial soup" on the earth 3.8 GY ago? Did life arise from that soup of organic compounds in single step or through a few intermediate stages? Scientists are trying to understand the possible intermediate steps in the origin of life in the hope that one day in the near future they can produce in the laboratory a living form from basic organic molecules.